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History Of Earthquakes In Cascadia

Cascadia has a rich and diverse history of earthquake activity, from the magnitude 7.5 event thought to have occurred in 900 A.D. along the Seattle Fault, to the more recent Klamath Falls (1993) and Nisqually (2001) quakes, to the hundreds of small quakes that occur every year but go mostly unfelt throughout the region. Earthquakes have struck offshore and inland, in small towns and busy city centers, in British Columbia, Northern California, Oregon and Washington.

Many recent earthquakes that grabbed headlines occurred outside of Cascadia, in countries such as Japan (Tohoku, 2011), New Zealand (Christchurch, 2011), Chile and Haiti (2010) and Indonesia (Sumatra, 2004). These far-away events, while not a part of Cascadia’s history, can teach us what to expect from future earthquakes in our region and can help us prepare and plan for them. Learn more about earthquakes in the news.

Cascadia subduction zone – a regional threat

Different parts of Cascadia experience earthquakes differently (see below), in part because earthquakes arise from different sources. But the entire region shares the need to plan for a Cascadia subduction zone earthquake. The Cascadia subduction zone lies offshore from northern California to southwestern British Columbia, where two tectonic plates -- the North America plate and the Juan de Fuca plate -- come together to form an 800-mile long earthquake fault.

Scientists believe the most recent subduction zone earthquake, a M9 event, occurred in January 1700. The best available evidence indicates that these earthquakes occur, on average, every 500 to 600 years. However, the years between these events have been as few as 100 to 300 years -- meaning, all Cascadia residents should prepare to experience a powerful and potentially damaging subduction zone earthquake in their lifetimes.

British Columbia

The Pacific coast forms the western border of British Columbia and is the most earthquake-prone region of Canada. Each year more than 1,000 earthquakes are recorded in western Canada. In the offshore region to the west of Vancouver Island, more than 100 earthquakes of magnitude 5 or greater (large enough to cause damage had they been closer to land) have occurred during the past 70 years. This concentration of earthquakes is related to the presence of active faults, or breaks in the Earth's crust, as well as several types of tectonic plate movements. Moving inland from the Pacific coast and the active plate boundaries, earthquakes decrease in frequency and size.

Nearly 17,000 earthquakes of magnitude 1 to 6 have been recorded in Oregon and Washington since 1970. About 15 to 20 quakes a year are felt in the Northwest.

Most small and moderate sized earthquakes in Oregon and Washington occur in the highly-populated Puget Sound region. More than 1,000 earthquakes occur in Washington each year. A dozen or more are strong enough that people feel ground shaking; occasionally, earthquakes cause damage. While most earthquakes occur in western Washington, some damaging events have occurred east of the Cascades, such as the M6.8 (est.) earthquake in 1872.

A note about Portland: Six earthquakes of M5 or greater have occurred within the greater Portland area in historical times, including the damaging M5.5 Portland earthquake (1962) and M5.6 Scotts Mills earthquake (1993). At least three crustal faults beneath the Portland metropolitan area could generate more damaging earthquakes of M6.5 or larger.

Earthquakes have affected all areas of northern California in the past and will do so again. Since 1900, nearly 40 earthquakes of magnitude 6 or larger have occurred north of Santa Rosa and in the adjacent offshore areas. The most damaging earthquakes in California in the past 150 years, including the 1906 San Francisco earthquake, have occurred in the San Andreas fault system that extends from northern California into southern California. San Andreas tremors continue to pose a risk to the state, but other seismic zones also are capable of producing earthquakes in the region as large or larger than the 1906 event. (Source: Humboldt State University)